The present invention relates generally to a motion snubber and more particularly to one suitable for use between the main rotor hub and mast of a teeter rotor helicopter.
Teeter rotors have two linearly aligned blades which teeter or wobble as they spin due to aerodynamic forces of flight. Technically, this is known as "flapping", a term which can be defined as the tilting of the plane of rotation of the rotor blades relative to the helicopter. Unlike other known rotor systems presently used, the teeter rotor's hub also flaps, sometimes dangerously close to the mast which links it to the helicopter. Under certain extreme conditions the spinning rotor hub flaps too far and cuts into the mast. This can and usually does have catastrophic consequences.
Mast bumping by the rotor can best be explained more fully with reference to the illustrations of FIGS. 1A through 1C which depict a mast bumping sequence.
As shown in FIG. 1A, during normal flight the free-floating main rotor 10, including blades 12 and 14 and hub 16, spins with a vertical flapping component of motion. A safety clearance exists between the rotor hub 16 and the mast 18. Teeter rotors and masts are designed to accommodate, for example, plus or minus five degrees of flap, but normally remain within much narrower flight limits, for example, plus or minus four degrees. Near the boundaries of the flight envelope, operational limits of flight set by the helicopter manufacturer, the flapping can result in the base of the main rotor 10 exceeding these angles and actually contacting the mast 18 as in FIG. 1B. Severe or repeated bumping of the hollow mast by the rotor 10 dents or crimps it. Such deformation is illustrated in FIG. 1C. Once mast bumping begins an unstable condition results in which the mast 18 is bumped repeatedly, first on one side and then the other. Some have even surmised that this can lead to a fracturing of the mast 18.
Various techniques to avoid or lessen the consequences of mast bumping have been suggested. These include: refitting the teeter rotor helicopter with three or four blades hinged to the rotor hub, providing thicker-walled masts, equipping the aircraft with gauges such as a G-meter and sideslip indicator to alert the pilot of the onset of unsafe flight conditions, and/or utilizing a device which can variously be called a motion snubber, bumper or hub spring between the rotor hub and the mast to restrain flapping.
The use of motion snubbers appears to be a viable technique. The motion snubber would serve to restrain flapping by resiliently cushioning the impact of the rotor hub. With appropriate design of the motion snubber, the pilot of the aircraft might be able to "feel" the impact of the rotor hub against the motion snubber and thus, act to avoid a major mishap.
Desirable would be a motion snubber designed so that it can be accommodated on teeter rotor helicopters without substantial redesign of the helicopter. Full rotor hub replacement to accommodate motion snubbers would be extremely costly. Preferable would be a snubber design which could be an "add-on" to existing rotor hub designs and yet meet the severe space envelope, motion accommodating and load carrying requirements which an add-on refit would entail.